Many satellite attitude reference and control systems require accurate, high bandwidth data indicating the angular velocity, that is vehicular rate of rotation of a main module or body of a satellite, commonly called the bus. The bus is the main portion of the satellite to which articulated appendages such as solar arrays, antenna and payloads are attached. The rotational angular velocity of the bus will change if the satellite carries gimballed appendages that react against the bus when the appendages are repositioned, typically during operational use. These changes are sensed by angular velocity measurement devices, commonly called gyroscopes, that are often used to measure the bus rotational angular velocity to provide rate data as required by reference and control systems on these satellites. The total momentum of a satellite will not change unless it is acted upon by an external torque under the principal of conservation of momentum. The total momentum of the satellite is the sum of the momentum in the bus and the appendages and momentum storage devices such as reaction wheels. In addition to attached appendages, a modern satellite system includes momentum storage devices such as a set of three reaction wheels providing reaction wheel tachometer data to an attitude control system for controlling the orientation and angular velocity rate of the satellite. The reaction wheels provide control torques to the vehicle bus while the attitude control system stores momentum in the reaction wheels from the external torques applied to the bus while generating reaction wheel tachometer data. The appendages are positioned by stepper motors. The stepper motors are considered control actuators that produce a controlled positioning rate to the appendage relative to the bus. A control input to the stepper motor will produce a given relative angular velocity between the appendage and the bus. The set of reaction wheels is used to control the bus attitude in all three rotational degrees of freedom. The reaction wheels provide control torques to the vehicle bus and store momentum from external torques applied to the bus by the external environment. The external torque is derived from natural disturbances or from on-board thrusters. The momentum of the bus will vary as the appendages react against the bus when the appendages are reoriented.
The gyro is used to provide vehicular bus rotational rates, that is, the angular velocity data used for vehicular attitude reference and attitude control. In order to accomplish controlled attitude orientation and controlled angular velocity of the satellite, the control system requires accurate attitude data from the reference system and accurate angular velocity rate data from the gyro. The gyro operation is subject to errors and biases that are corrected by a low frequency filter such as a Kalman filter in the attitude reference system. The low frequency filter is any filter that receives measurement data and produces update data with a low frequency bandwidth within a high bandwidth of the system operation. Hence, the gyro is an integral and essential part of an attitude reference system and is necessary for the proper functioning of the attitude control system. Modern applications require accurate, high bandwidth gyroscopes with long lifetimes, high reliability and low power usage.
Appendages typically have encoders that measure appendage position relative to the bus and are positioned by stepper motors. The stepper motors are considered control actuators that produce a controlled rate to the appendage relative to the bus. An appendage control system is used to control a set of appendages. The appendage control system uses appendage measurements usually generated by encoders or resolvers on the shaft of the appendages that are typically solar arrays and antenna and payload type appendages. The appendage measurements usually include angular position data and sometimes the angular velocity data. These measurements are included for all appendages having significant momentum contributions to the system momentum. There are multiple appendages that contribute significantly to the system momentum, that is, there are multiple appendage measurements that can be used to significantly indicate the system momentum. For example, a typical satellite may use three appendages, such as a communication antenna assembly and two solar panels. Other appendages, including payload, optical, RF and IR sensors, can be used by the satellite. A communications antenna assembly, has two degrees of freedom. A stepper motor connected to a harmonic drive gear assembly actuates each degree of freedom. The input to the stepper motors is an angular rate command in terms of motor steps per second. Because each stepper motor accurately produces the commanded rate, the commanded rate is taken to be the true angular rate of the stepper motor. The angular rate of the appendage is the commanded rate of the stepper motor times the gear reduction ratio of the harmonic drive. This angular rate for both degrees of freedom is commanded by the appendage control system. Optical encoders measure the angular position of each of the degrees of freedom. The measurement data from the encoder is converted to radians and sent to the appendage control system as an angular position. The other two appendages are solar array panels each containing two degrees of freedom. As with the communications antenna assembly, stepper motors actuate both panels and encoders measure their angular position. Each panel has two stepper motors and two encoders. The four stepper motor commands and four encoder measurements are processed in the appendage control system.
A typical reference system of a spacecraft uses star trackers and gyro operations to accurately determine both current vehicular attitude and current angular velocity. The typical attitude reference system receives star catalog ground update data into a star tracking processor for providing a Kalman filter with predicted star orientation while a star tracker provides the actual star orientation. The difference between the predicted and actual star orientation is the residual error input that drives the Kalman filter. The star tracker is a sensor that measures the angular orientation of visual stars within an optical field of view of the sensor and provides the star tracking processor with actual star orientation data and provides the Kalman filter with measured star vectors indicating the orientation of the star in the sensor field of view. The star tracker searches for stars in the sensor field of view and compares the located star to the identified stars from the star catalog. The Kalman filter also receives vehicular rate data from the gyro and receives vehicular attitude data from an attitude propagator. The rate bias update data and attitude update data generated by the Kalman filter are communicated to the gyro and an attitude propagator, respectively. The propagated attitude is the onboard estimate of the vehicular orientation in the earth centered inertial coordinates frame.
Some reference systems may use multiple star trackers with each star tracker providing accurate positional information of various stars being tracked. Specifically, each star tracker produces horizontal and vertical outputs forming a unit vector that points in the direction of a tracked star. These horizontal and vertical outputs are provided at an update rate of the star tracker. In a typical case, the update rate of the star trackers is about 1.5 hertz. The star tracking processor uses the star tracker outputs to compute the error between the measured star vectors and the predicted star vectors. This computed measurement error is then used by the Kalman filter to generate attitude updates for updating the vehicular attitude and rate bias updates for the gyro.
The first step of a star tracking processor is star identification. In order to produce a measurement error a tracked star must be identified by the star tracking processor. A star catalog is uploaded in a spacecraft computer for access by the star tracking processor. The star catalog contains the positional information and brightness measurements for hundreds of stars. The positional information is stored in earth centered inertial coordinates. The brightness information is a relative measurement of how bright the star should appear to the star tracker. Because of the proper motion of stars and the precession of the earth centered inertial coordinate frame, the star catalog is updated periodically from a ground location.
In order to identify a star, the location of a tracked star is transformed to earth centered inertial coordinates. This is done by forming a measurement vector in a star tracker coordinate frame from the horizontal and vertical star tracker outputs. The measurement vector is rotated into the earth centered inertial frame using a transformation between the star tracker and earth centered inertial coordinates that includes the propagated vehicular attitude. Once the location of a star in earth centered inertial coordinates has been calculated, coordinates are compared to the coordinates of the identified star in the star catalog. When the calculated coordinates match the coordinates of the identified star in the catalog within a specified tolerance and the brightness matches that of the star in the catalog within a specified tolerance the star has been identified. When a star has been identified, then the star tracking processor can compute the star measurement error for the Kalman filter. When the propagated vehicular attitude is correct, the measured vector from the star tracker will be oriented in the same direction as the identified star from the star catalog. When there is an error in the propagated attitude data 24, the residual between the catalog orientation and the measured star orientation is used by the Kalman filter to update the propagated attitude.
The on-board Kalman filter estimates six state variables including three attitude orientation errors, and three angular velocity rate errors. The attitude errors are used to provide a vehicular attitude update, and the angular velocity rate errors are used to update the bias of the gyro. The gyro bias is the difference between the gyro output and the true rate of rotation of the gyro, and the gyro bias is needed to compensate for errors in the gyro vehicular rate output. After the Kalman filter processes the star measurement error data into an attitude update, and a gyro bias error update, the states of the Kalman filter, the Kalman gain, and covariance matrix are updated using the discrete Kalman filter equations as is well known in the art. An attitude propagator is used to calculate the vehicular attitude between star tracker updates. The vehicular angular velocity rates from the gyro are multiplied by time since the last attitude propagation to give a small current angular rotation of the satellite specified by a small attitude transformation matrix indicating a small angle of rotation. The previous vehicular attitude is updated by this small attitude transformation matrix of rotation of the satellite to give the propagated vehicular attitude.
The attitude control system also functions to control the momentum of the spacecraft. Effects by external torques are measured by the gyro and include torques due to a gravity gradient. Gravity gradient torques are caused by the earth gravity by acting to align the vehicular principal axes of inertia, with the gravity vector directed towards the center of the earth. Well-known gravity gradient torques are dependent on the vehicular attitude relative to the gravity vector extending from the earth. Torque rods conduct current for generating external torque by magnetically interacting with the magnetic field of the earth. The attitude control system generates torque rod current used to transfer unwanted momentum from the satellite into the earth magnetic field.
The appendage measurement data, reaction wheel tachometer data and external torque rod current data define changes in momentum of the bus. These appendage measurement data, reaction wheel tachometer data, and external torque rod current data are momentum data related to the momentum of the spacecraft and processed by the attitude control system. This momentum data is available for momentum computational purposes, but has not been previously used in a reference system to compute momentum values of the spacecraft nor used to emulate a mechanical gyro. Gyros have accordingly long been used successfully in satellite attitude reference systems to provide the vehicular angular velocity rates of the satellite for satellite attitude referencing and positioning control. However, gyros are expensive and inherently have limited life times with low reliability and dynamic error characteristics, leading to premature failure of satellite systems. These and other disadvantages are solved or reduced using the invention.